Weird

So my beautiful baby girl will be 7 months on the 21st.....she can crawl and she can pull herself up to stand. She can even sit on her knees and balance with no hands (like WHAAAA) but still has yet to sit on her own.

I need some opinions. When would be the time, if had to, to get her looked at to make sure she is ok. I know sitting is about back strength and balance, but she is strong enough to balance on knees.

I'm not worried yet, but I want to make sure I get her looked at, if need be, before it would be a problem.